As an ophthalmologic apparatus for examining an eye retina, there is known a scanning laser ophthalmoscope (SLO) which obtains a two-dimensional image of a retina as a plane. In addition, there is also known an optical coherence tomography (OCT) which obtains a tomographic image of a retina in a noninvasive manner. Those apparatuses have been put into practical use for a long time. Those apparatuses scan a retina by a light beam in a two-dimensional manner using a deflector, and synchronously measure reflected and backscattering light so as to obtain a two-dimensional image or a three-dimensional image of the retina.
Spatial resolution of the obtained image in a plane direction (lateral direction) of the retina (hereinafter, referred to as lateral resolution) is determined basically by a spot diameter of the beam that scans the retina. In order to reduce the spot diameter of the beam condensed on the retina, only the diameter of the beam that enters the eye needs to be increased. However, curved surface shapes and refractive index homogeneity of cornea and lens that perform the refracting action in an eyeball are not complete, which causes high order aberration in a wavefront of the transmitted light. Therefore, even if a thick beam enters, the spot on the retina cannot be condensed to a desired diameter and, on the contrary, is expanded. As a result, the lateral resolution of the obtained image is decreased, and the S/N ratio of the obtained image signal is also decreased. Therefore, conventionally, it has been common to use incident light of a thin beam of approximately 1 mm diameter that is hardly affected by aberration of the eye's optical system, and to form a spot of approximately 20 μm diameter on the retina.
A structure that is generally and often used in the two-dimensional scan optical system such as SLO or OCT is the structure in which two one-dimensional scanner mirrors are disposed close to each other (hereinafter, referred to as a tandem type). PTL 1 discloses a structure in which galvano mirrors, each of which can rotate about a rotation axis, are disposed close to each other so that the rotation axes are perpendicular to each other, and the incident beam is deflected in the two-dimensional direction so as to enter the pupil of the eyeball via an imaging optical system and an objective lens. In addition, NPL 1 discloses a structure using a two-dimensional scanner having a similar structure, in which a thick beam of 3.68 mm diameter is formed as the incident beam of the eyeball, and a compensation optical system using a wavefront sensor and a variable shape mirror is also disposed for correcting influence of aberration of the eyeball optical system.